Control system for hybrid vehicles

ABSTRACT

A control system for a hybrid vehicle, in which an engine is connected to a first motor generator and to an output shaft through a power distributing mechanism ( 12 ), in which a second motor generator is connected to the output shaft through a transmission ( 6 ) wherein a torque capacity is varied in accordance with an oil pressure, and which has an electric oil pump (OPM) for generating an oil pressure, comprising: an oil pressure judging means for judging whether or not the oil pressure established by operating OPM is raised higher than a preset value; an electric oil pump output lowering device for lowering an output of OPM in case the oil pressure judging device judges that the oil pressure is raised higher than the preset value; and a cranking device for carrying out a cranking of E/G by the first motor generator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a control system for a hybridvehicle provided with a plurality of prime movers for running a vehicle.More particularly, to a control system for a hybrid vehicle, which hasan electric oil pump for generating an oil pressure to set a torquecapacity of a power transmission system.

The disclosure of Japanese Patent Application No. 2004-14316 filed onJan. 22, 2004 including specification, drawings and claims isincorporated herein by reference in its entirety.

2. Discussion of the Related Art

One example of the hybrid vehicle equipped with a “mechanicaldistribution type drive unit” is disclosed in Japanese Patent Laid-OpenNo. 2002-225578, and a construction thereof will be described brieflyhereafter. In the disclosed hybrid vehicle, an engine torque is inputtedto a carrier of a single pinion type planetary gear mechanism whichconstitutes a distributing mechanism, a first motor generator isconnected to a sun gear, and an output member such as a counter drivegear etc. is connected to a ring gear. A second motor generator isconnected to the output member or the ring gear through a transmission.The transmission is capable of switching a gear stage between a directgear stage in which the entire transmission rotates integrally, and alow gear stage in which an output speed is lower than an input speed.Those gear stages are set by properly operating an engagement mechanismoperated by the oil pressure.

The hybrid vehicle of this kind can be run not only by a motive power ofan engine and the first motor generator, but also by using a torqueoutputted from the second motor generator as an assist torque, or onlyby an output torque of the second motor generator.

In order to ensure the oil pressure when the engine is halted, in thehybrid vehicle disclosed in Japanese Patent Laid-Open No. 2002-225578,it is conceivable to further provide an electric hydraulic pump which iscapable of generating the oil pressure even when the engine is halted,in addition to a hydraulic pump which is driven by the engine. In thishybrid vehicle, moreover, the engine is connected to the first motorgenerator through a planetary gear mechanism, so that the engine can bestarted by carrying out a cranking (or a motoring) by the first motorgenerator. In this case, since an output shaft is connected to theplanetary gear mechanism in addition to the engine and the first motorgenerator, torque acts on the output shaft in the direction to rotate itbackward when cranking the engine by the first motor generator. For thisreason, a “backward torque”, which appears on the output shaft whencarrying out a cranking by the first motor generator is countervailed byoutputting the torque to the output shaft by the second motor generator.As a result of this, it is possible to prevent a vibration in thevehicle and a backward movement of the vehicle.

On the other hand, the transmission is arranged between the second motorgenerator and the output shaft. In order to transmit the torque from thesecond motor generator to the output shaft, therefore, the transmissionhas to have a predetermined torque capacity. In this case, the enginehas not yet been started so that it is impossible to obtain the oilpressure from the oil pump driven by the engine. Accordingly, it isnecessary to generate the oil pressure by operating an electric oil pumpprovided in addition to the oil pump driven by the engine. As a resultof this, both the first motor generator for starting the engine and theelectric oil pump are driven at a starting time of the engine. If bothof those first motor generator and electric oil pump are drivensimultaneously, a load on an accumulator device such as a batteryincreases. This results in a shortage of electric power supply to thefirst motor generator or the like. Consequently, a cranking torquebecomes insufficient so that it may take long time to startup theengine.

SUMMARY OF THE INVENTION

An object of this invention is to prevent a delay in startup of a hybridvehicle comprising a plurality of prime movers including an internalcombustion engine, and an electric oil pump for generating an oilpressure to set a torque capacity of a transmission.

In order to achieve the aforementioned object, according to the presentinvention, there is provided a control system for a hybrid vehicle,characterized in that: an electric motor for rotating the internalcombustion engine to start, and the electric oil pump for generating anoil pressure to set a torque capacity of the transmission, arecontrolled correlatively to each other. More specifically, according tothe present invention, there is provided a control system for a hybridvehicle, in which an internal combustion engine is connected to a firstelectric motor and to an output member through a power distributingmechanism, in which a second electric motor is connected to the outputmember through a transmission wherein a torque capacity is varied inaccordance with an oil pressure, and which has an electric oil pump forgenerating an oil pressure to set the torque capacity of thetransmission, comprising: an oil pressure judging means for judgingwhether or not the oil pressure established by operating the electricoil pump is raised higher than a preset value; an electric oil pumpoutput lowering means for lowering an output of the electric oil pump incase the oil pressure judging means judges that the oil pressure israised higher than the preset value; and a cranking means for carryingout a cranking of the internal combustion engine by the first electricmotor.

According to the present invention, therefore, the output of theelectric oil pump is lowered, and the first electric motor carries outthe cranking of the internal combustion engine, in case the oil pressuregenerated by the electric oil pump is raised higher than the presetvalue. Accordingly, it is possible to avoid a situation where theelectric oil pump and the first electric motor are driven simultaneouslyto the high output, so that the cranking of the internal combustionengine can be executed by feeding sufficient electric power to the firstelectric motor. Consequently, a prompt startup of the internalcombustion engine can be achieved.

In addition to the above-mentioned construction, a control system for ahybrid vehicle of the present invention further comprises: a secondelectric motor control means for carrying out a constant-speed controlto keep the speed of the second electric motor at constant speed in theprocess of raising the oil pressure by operating the electric oil pump;and characterized in that the oil pressure judging means includes ameans for determining the oil pressure on the basis of the change in thespeed of the second electric motor which is controlled to the constantspeed.

With this construction, the torque capacity of the transmission isincreased in accordance with a rise in the oil pressure, and on theother hand, the output member is fixed. Therefore, the speed of thesecond electric motor controlled to the constant speed is varied inaccordance with the rise in the oil pressure, and this speed change isdetected to judge the rise in the oil pressure. Consequently, the risein the oil pressure established by the electric oil pump can be detectedon the basis of the change in the speed of existing second motorgenerator, without providing new equipment such as an oil pressuresensor.

In addition, a complete combustion of the internal combustion engine canbe determined on the basis of a current value or speed of the firstelectric motor.

According to the invention, moreover, the oil pressure judging means canincludes a means for judging whether or not a preset time period, inwhich the oil pressure resulting from the operation of electric oil pumpdoes not exceed a preset value, has been elapsed; the electric oil pumpoutput lowering means can include a means for lowering the output of theelectric oil pump in case a lapse of the preset time period is judged;and the cranking means can include a means for carrying out the crankingof the internal combustion engine by the first electric motor after thelapse of the preset time is judged by the oil pressure judging means.

With this construction, therefore, the output of the electric oil pumpis lowered and the cranking of the internal combustion engine is carriedout by the first electric motor, in case the preset time period, inwhich the oil pressure resulting from the operation of the electric oilpump does not exceed a preset value, has been elapsed. Accordingly, itis possible to avoid a situation where the electric oil pump and thefirst electric motor are driven simultaneously to the high output, andit is also possible to avoid a situation where the electric oil pump iskept driving excessively so that the electric power is highly consumed.

According to the invention, furthermore, it is possible to provide afixing mechanism for fixing the output member at the startup of theinternal combustion engine. Providing the fixing mechanism prevents aparticular change of a behavior of the vehicle, even in case theinternal combustion engine is started before the oil pressure of theelectric oil pump is raised sufficiently.

The above and further objects and novel features of the invention willmore fully appear from the following detailed description when the sameis read with reference to the accompanying drawings. It is to beexpressly understood, however, that the drawings are for purpose ofillustration only and are not intended as a definition of the limits ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart for explaining a control example by a controlsystem of this invention.

FIG. 2 is a time chart for explaining a control example by a controlsystem of this invention.

FIG. 3 is a diagram schematically showing a drive unit of a vehicle towhich the invention is applied.

FIG. 4 is a diagram showing a drive unit of a vehicle to which theinvention is applied.

FIG. 5 is a diagram showing a hydraulic control circuit to which theinvention is applied.

FIG. 6 is a nomographic diagram on the drive unit to which the inventionis applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention will be described in connection with its specificexamples. The first description is made on a drive unit of a hybridvehicle, to which is this invention is applied. In the hybrid drive unitor an application target of this invention, as shown in FIG. 3, thetorque of a main prime mover 1 (i.e., a first prime mover) istransmitted to an output member 2, from which the torque is transmittedthrough a differential 3 to drive wheels 4. On the other hand, there isprovided an assist prime mover (i.e., a second prime mover) 5, which canmake a power control to output a driving force for a drive and aregenerative control to recover an energy. This assist prime mover 5 isconnected through a transmission 6 to the output member 2. Between theassist prime mover 5 and the output member 2, therefore, thetransmission torque capacity is increased/decreased according to a gearratio to be set by the transmission 6.

This transmission 6 can be constructed to set the gear ratio at “1” orhigher. With this construction, at the power running time for the assistprime mover 5 to output the torque, this torque can be outputted to theoutput member 2 so that the assist prime mover 5 can be made to have alow capacity or a small size. However, it is preferred that the runningefficiency of the assist prime mover 5 is kept in a satisfactory state.In case the speed of the output member 2 rises according to the vehiclespeed, for example, the gear ratio is lowered to decrease the speed ofthe assist prime mover 5. In case the speed of the output member 2drops, on the other hand, the gear ratio may be raised.

The aforementioned hybrid drive unit will be described morespecifically. As shown in FIG. 4, the main prime mover 1 is mainlyconstructed to include an internal combustion engine 10 (as will becalled the “engine”), a motor generator (as will be tentatively calledthe “first motor generator” or “MG 1”) 11, and a planetary gearmechanism 12 for synthesizing or distributing the torque between thoseinternal combustion engine 10 and first motor generator 11. The engine10 is a well-known power unit such as a gasoline engine or a dieselengine for outputting a power by burning a fuel, and is so constructedthat its running state such as the degree of throttle opening (or theair intake amount), the fuel feed amount or the ignition timing can beelectrically controlled. This control is made by an electronic controlunit (E-ECU) 13 composed mainly of a microcomputer, for example.

On the other hand, the first motor generator 11 is exemplified by apermanent magnet type synchronous electric motor and is constructed tofunction as an electric motor and a dynamo. The first motor generator 11is connected through an inverter 14 with an accumulator device 15 suchas a battery. By controlling the inverter 14, moreover, the outputtorque or the regenerative torque of the first motor generator 11 issuitably set. For this control, there is provided an electronic controlunit (MG1-ECU) 16, which is composed mainly of a microcomputer. Here, astator (a stator not shown) of the first motor generator 11 is so fixedas not to rotate.

Moreover, the planetary gear mechanism 12 is a well-known one forestablishing a differential action with three rotary elements: a sungear 17 or an external gear; a ring gear 18 or an internal gear arrangedconcentrically with the sun gear 17; and a carrier 19 holding a piniongear meshing with those sun gear 17 and ring gear 18 such that thepinion gear may rotate on its axis and revolve around the carrier 19.The engine 10 has its output shaft connected through a damper 20 to thatcarrier 19 as a first rotary element. In other words, the carrier 19acts as an input element.

On the other hand, a rotor (rotor not shown) of the first motorgenerator 11 is connected to the sun gear 17 as a second rotary element.Therefore, this sun gear 17 is the so-called “reaction element”, and thering gear 18 as a third rotary element is the output element. And, thisring gear 18 is connected to the output member (i.e., the output shaft)2.

In the example shown in FIG. 4, on the other hand, the transmission 6 isconstructed of one set of Ravignaux type planetary gear mechanisms. Theplanetary gear mechanism is provided with external gears, i.e., a firstsun gear (S1) 21 and a second sun gear (S2), of which the first sun gear21 meshes with a first pinion 23, which meshes with a second pinion 24,which meshes with a ring gear (R) 25 arranged concentrically with theindividual sun gears 21 and 22. Here, the individual pinions 23 and 24are so held by a carrier (C) 26 as to rotate on their axes and torevolve around the carrier 26. Moreover, the second sun gear 22 mesheswith the second pinion 24. Thus, the first sun gear 21 and the ring gear25 construct a mechanism corresponding to a double-pinion type planetarygear mechanism together with the individual pinions 23 and 24, and thesecond sun gear 22 and the ring gear 25 construct a mechanismcorresponding to a single pinion type planetary gear mechanism togetherwith the second pinion 24.

There are also provided a first brake B1 for fixing the first sun gear21 selectively, and a second brake B2 for fixing the ring gear 25selectively. These brakes B1 and B2 are the so-called “frictionalengagement devices” for establishing engaging forces by frictionalforces, and can adopt a multi-disc engagement device or a band typeengagement device. The brakes B1 and B2 are constructed to change theirtorque capacities continuously according to the engaging forces of oilpressures. Moreover, the aforementioned assist prime mover 5 isconnected to the second sun gear 22, and the carrier 26 is connected tothe output shaft 2. Furthermore, a parking gear 37 for putting thevehicle into a parking state is installed on the output shaft 2. Also,there is provided a parking lock pawl 38 for halting a rotation of theparking gear 37 by engaging therewith, in case a parking position isselected by a not shown shifting device.

In the transmission 6 thus far described, therefore, the second sun gear22 is the so-called “input element”, and the carrier 26 is the outputelement. The transmission 6 is constructed to set high gear stages ofgear ratios higher than “1” by applying the first brake B1, and to setlow gear stages of gear ratios higher than those of the high gear stagesby applying the second brake B2 in place of the first brake B1. Theshifting operations between those individual gear stages are executed onthe basis of a running state such as a vehicle speed or a drive demand(or the degree of accelerator opening). More specifically, the shiftingoperations are controlled by predetermining gear stage regions as a map(or a shifting diagram) and by setting any of the gear stages accordingto the detected running state. For these controls, there is provided anelectronic control unit (T-ECU) 27, which is composed mainly of amicrocomputer.

Here in the example shown in FIG. 4, there is adopted a motor generator(as will be tentatively called the “second motor generator” or “MG2”) asthe assist prime mover 5, which can have the power mode to output thetorque and the regenerative mode to recover the energy. This secondmotor generator 5 is exemplified by a permanent magnet type synchronouselectric motor and its rotor (rotor not shown) is connected to thesecond sun gear 22. Also, the second motor generator 5 is connectedthrough an inverter 28 with a battery 29. Moreover, the motor generator5 is constructed to control the power mode, the regenerative mode andthe torques in the individual modes by controlling the inverter 28 withan electronic control unit (MG2-ECU) 30 composed mainly of amicrocomputer. Here, the battery 29 and the electronic control unit 30can also be integrated with the inverter 14 and the battery (theaccumulator device) 15 for the aforementioned first motor generator 11.Additionally, a stator (a stator not shown) of the second motorgenerator 5 is so fixed as not to rotate.

A nomographic diagram of the single pinion type planetary gear mechanism12 as the aforementioned torque synthesizing/distributing mechanism ispresent at (A) in FIG. 6. When the reaction torque by the first motorgenerator 11 is inputted to the sun gear (S) 17 against the torque to beinputted to the carrier (C) 19 and outputted by the engine 10, a torquein the magnitude derived from an addition or subtraction of those torqueappears at the ring gear (R) 18 acting as the output element. In thiscase, the rotor of the first motor generator 11 is rotated by thistorque, and the first motor generator 11 functions as a dynamo. With thespeed (or the output speed) of the ring gear 18 being constant, on theother hand, the speed of the engine 10 can be continuously (or withoutany step) changed by increasing/decreasing the speed of the first motorgenerator 11. Specifically, the control for setting the speed of theengine 10 at a value for the best fuel economy can be made bycontrolling the first motor generator 11.

As indicated by a chain line in FIG. 6(A), moreover, the first motorgenerator 11 rotates backward when the engine 10 is halted while thevehicle is running. In this state, if the torque is outputted in aforward direction by operating the first motor generator 11 as theelectric motor, the torque acts on the engine 10 connected to thecarrier 19 to rotate it in the forward direction. As a result, theengine 10 can be started (i.e., motored or cranked) by the first motorgenerator 11. In this case, the torque acts on the output shaft 2 in thedirection to stop the rotation of the output shaft 2. Therefore, thedriving torque for running can be maintained by controlling the torqueoutputted from the second motor generator 5, and at the same time, thestartup of the engine 10 can be executed smoothly. Here, the hybrid typeof this kind is called “mechanical distribution type” or “split type”.

On the other hand, a nomographic diagram of the Ravignaux type planetarygear mechanism constructing the transmission 6 is presented at (B) inFIG. 6. When the ring gear 25 is fixed by the second brake B2, a lowgear stage L is set so that the torque outputted from the second motorgenerator 5 is amplified according to the gear ratio and applied to theoutput shaft 2. When the first sun gear 21 is fixed by the first brakeB1, on the other hand, there is set a high gear stage H having a lowergear ratio than that of the low gear stage L. The gear ratio at thishigh gear stage is higher than “1” so that the torque outputted by thesecond motor generator 5 is augmented according to that gear ratio andapplied to the output shaft 2.

Here, in the state where the individual gear stages L and H are steadilyset, the torque to be applied to the output shaft 2 is such one as isaugmented from the output torque of the second motor generator 5according to the gear ratio. In the shifting transitional state,however, the torque is such one as is influenced by the torquecapacities at the individual brakes B1 and B2 and by the inertia torqueaccompanying the speed change. On the other hand, the torque to beapplied to the output shaft 2 is positive in the drive state of thesecond motor generator 5 but negative in the driven state.

There is provided a hydraulic control system 31 for controlling theengagement/release of the aforementioned individual brakes B1 and B2 byfeeding/discharging the oil pressure thereto/therefrom. As shown in FIG.5, the hydraulic control system 31 comprises a mechanical oil pump 32,an electric oil pump 33, and a hydraulic circuit 34. The hydrauliccircuit 34 is constructed to regulate an oil pressure established bythose oil pumps 32 and 33 to a line pressure, to feed and discharge theoil pressure regulated from the line pressure as an initial pressureto/from the brakes B1 and B2, and to feed a lubrication oil to requiringportions. The mechanical oil pump 32 is driven by the engine 10 togenerate the oil pressure, and is arranged e.g., in an output side ofthe damper 20 and coaxially therewith. The mechanical oil pump 32 isoperated by the torque of the engine 10. On the other hand, the electricoil pump 33 is driven by a motor 33M, and is arranged at a suitableplace such as an outside of a casing (casing not shown). The electricoil pump 33 is operated by an electric power from an electricaccumulator such as a battery to generate oil pressure.

The hydraulic circuit 34 comprises a plurality of solenoid valves,change-over valves or pressure regulator valves (those are not shown),and the regulation and the feeding/discharging of the oil pressure canbe controlled electrically. Here, there are provided check valves 35 and36 on a discharging side of the individual oil pumps 32 and 33. Thosecheck valves 35 and 36 are opened by a discharging pressure of those oilpumps 32 and 33, and closed in an opposite direction. The oil pumps 32and 33 are connected to the hydraulic circuit 34 and those pumps arearranged in parallel with each other. In addition, a valve forregulating the line pressure (valve not shown) controls the linepressure into two stages, such as a high pressure stage by increasingthe discharging amount, and a low pressure stage by decreasing thedischarging amount.

The aforementioned hybrid drive unit comprises two prime movers such asthe main prime mover 1 and the assist prime mover 5. The vehicle runs atlow fuel consumption and at low emission by making good use of thoseprime movers. Even in case of driving the engine 10, moreover, the speedof the engine 10 is controlled at the optimum fuel consumption by thefirst motor generator 11. Moreover, inertia energy of the vehicle isregenerated as electric power at the coasting time. In case the torqueis assisted by driving the second motor generator 5, the torque to beadded to the output shaft 2 is augmented by setting the transmission 6at the low gear stage L when the vehicle speed is low. On the otherhand, the speed of the second motor generator 5 is relatively lowered toreduce the loss by setting the transmission 6 at the high gear stage Hwhen the vehicle speed is raised. Thus, the torque assist is carried outefficiently.

The aforementioned hybrid vehicle is capable of running by the power ofthe engine 10, by both of the engine 10 and the second motor generator5, and only by the second motor generator 5. Those running pattern isdetermined and selected on the basis of drive demands represented by theaccelerator opening, vehicle speed, and so on. For example, in case thebattery charge is sufficient and the drive demand is relatively small,or in case a quiet start is selected by a manual operation, the runningpattern is selected which is similar to that of an electric vehicle (aswill be tentatively called “EV running”) using the second motorgenerator 5, and the engine 10 is halted. In this state, in case thedrive demand is increased when e.g., the accelerator pedal is depresseddeeply, in case the battery charge is decreased, or in case the runningstate is shifted from the quiet start to a normal running by a manualoperation, the engine 10 is started and the running pattern is shiftedto the running pattern using the engine 10 (as will be tentativelycalled “E/G running”).

In the aforementioned example, a startup of the engine 10 is carried outby the first motor generator 11 functioning as the motor, and bytransmitting the torque to the engine 10 through the planetary gearmechanism 12 so as to carry out the motoring (or the cranking). In thiscase, if the torque is applied to the sun gear 17 by the first motorgenerator 11 in the direction to rotate the sun gear 17 forward, thetorque acts on the ring gear 18 in the direction to rotate the ring gear18 backward. Since the ring gear 18 is connected to the output shaft 2,the torque involved in the startup of the engine 10 acts in thedirection to reduce the vehicle speed. At the startup of the engine 10,therefore, the torque is outputted from the second motor generator 5 inorder to countervail a “reaction torque”.

In case of starting the vehicle from a state where a ignition switch isnot turned to a starting position, a start control is carried out tofeed the oil pressure by operating the electric oil pump 33. However,since the cranking of the engine 10 is carried out by the first motorgenerator 11, execution of the cranking is a load on the accumulatordevice such as the battery.

Since electric power is not generated by the motor generators 5 and 11when the start control is carried out, so that the operation of theelectric oil pump 33 is also a load on the accumulator device such asthe battery. Therefore, in order to avoid a reduction in the respectiveoutputs resulting from a competition for the electric power of theaccumulator device such as the battery, between the applications such asthe cranking of the engine 10 by the first motor generator 11 andoperation of the electric oil pump 33, the control system of theinvention carries out the following controls.

FIG. 1 is a flowchart for explaining an example of the control. First ofall, it is judged (at Step S1) whether or not the system is started up.Specifically, it is judged whether or not a start switch of the entirevehicle, e.g., the ignition switch is turned to the starting position.In case the answer of Step S1 is NO, the routine is ended.

In case the answer of Step S1 is YES, the electric oil pump (OPM) 33 isstarted and the output thereof is raised immediately to maximum (at StepS2). Here, the start switch of the entire vehicle is turned ON only inthe case where a sift position is in a parking position (P) or in aneutral position (N). In case of starting the engine 10 when the siftposition is in a parking position, the parking lock pawl 38 engages withthe parking gear 37 and the output shaft 2 is thereby fixed.

On the other hand, the second motor generator 5 is not directly involvedin the startup of the engine 10, if the torque capacity of thetransmission 6 is insufficient. However, a constant-speed control toraise the speed of the second motor generator 5 and keep it at constantspeed is carried out (at Step S3). The second brake B2 is engagedgradually as the oil pressure rises, and a negative torque acting on thesecond motor generator 5 is consequently increased so that the speed ofthe second motor generator 5 is lowered. Therefore, the rise in the oilpressure can be detected from a change in the speed of the second motorgenerator 5. This is a reason for raising the speed of the second motorgenerator 5 in advance.

Then, it is judged (at Step S4) whether or not a preset time has elapsedfrom the instant when the electric oil pump 33 has been started. Thispreset time is set in advance as a time period until the oil pressuregenerated by starting the electric oil pump 33 reaches the oil pressurecapable of setting a sufficient torque capacity of the transmission 6.In the beginning of the start control, sufficient time has not elapsedyet so that the answer of Step S4 is NO. In this case, a determinationof engagement of the brake B2 is made subsequently (at Step S5). Theengagement of the brake B2 can be determined from the fact that a speeddeference Δ N between the speeds of the brake B2 and that of anotherrotary member (e.g., the output shaft 2) becomes smaller than a presetvalue, or the fact that the speed of the second motor generator 5becomes smaller than a preset value.

The oil pressure still has not risen just after the startup of theelectric oil pump 33. Therefore, no changes come about in the speed ofthe second motor generator 5 so that the answer of Step S5 is NO. Inthis case, the routine is returned so as to continue the precedingcontrol. On the contrary, when the oil pressure rises to a certainlevel, changes come about in the speeds of the brake B2 and the secondmotor generator 5 so that answer of Step S5 is YES. Specifically, it isdetermined that the oil pressure established by operating the electricoil pump 33 is raised higher than the preset value, on the basis of thechange in the speed.

In case the answer of Step S5 is YES, the speed of the electric oil pump(OPM) 33 is lowered. That is, the output of the electric oil pump 33 islowered. Also, the cranking of the engine 10 is carried out by operatingthe first motor generator 11 as a motor (at Step S6). Accordingly, theoutput of the electric oil pump 33 is lowered at the instant when thefirst motor generator 11 is operated as a motor by supplying electricpower thereto, therefore, the electric power is ensured sufficiently. Asa result, the cranking of the engine 10 can be carried out certainly bythe first motor generator 11 so that the engine 10 can be startedpromptly.

The complete combustion of the engine 10 is determined while thecranking of the engine 10 is thus being carried out (at Step S7).Specifically, it is judged whether or not the engine 10 starts rotatingautonomously. When the engine 10 rotates autonomously, a negative torqueof the first motor generator 11 is lowered and turned into a positivetorque, so that a current value and speed of the first motor generator11 are changed. Therefore, the determination at Step S7 can be made onthe basis of the current value and speed of the first motor generator11.

In case the answer of Step S7 is NO, the routine is returned to Step S6so as to continue the cranking by the first motor generator 11. On thecontrary, in case the engine 10 is in a complete combustion condition sothat the answer of Step S7 is YES, the electric oil pump 33 is halted(at Step S8). This is because the mechanical oil pump 32 is driven bythe engine 10 and generates the oil pressure sufficiently when thestartup of the engine is completed.

After the electric oil pump 33 is started, the preset time has elapsedwithout any affirmative judgment at step S5 so that the answer of theStep S4 is YES, the routine advances to Step S6 immediately and anoutput lowering control for the electric oil pump 33 and the cranking ofthe engine 10 are carried out. Specifically, in case the affirmativejudgment has not been made at Step S5 within the preset time periodafter the electric oil pump 33 is started, it is presumed that the oilpressure would not be raised sufficiently due to some sort of troubles.Therefore, if the electric oil pump 33 is driven longer than the presettime period, the electric power may be consumed excessively. For thisreason, the electric oil pump 33 is not driven longer than the presettime period. In this case, the transmission 6 does not have enoughtorque capacity so that the torque of the second motor generator 5cannot be transmitted to the output shaft 2. Specifically, the torquewhich acts on the output shaft 2 as a result of the cranking of thesecond motor generator 5 cannot be canceled sufficiently by the torqueof the second motor generator 5. However, since the output shaft 2 isfixed by the aforementioned parking gear 37 and the parking lock pawl38, it is possible to receive a reaction force at the cranking time ofthe engine 10 by the parking gear 37 and the parking lock pawl 38.Consequently, the vehicle will not be moved backward.

Next, here will be described a temporal course of this example withreference to a time chart of FIG. 2. When the start switch of the entirevehicle, e.g., the ignition switch is turned to the starting positionand the start control is commenced (at a point of time t1, correspondingto Step S1), the electric oil pump 33 is started immediately andcontrolled to maximum output. Accordingly, the oil pressure generated bythe electric oil pump 33 rises with a time delay (from the point of timet1 to a point of time t2, corresponding to Steps S2 to S4), and anengaging pressure of the brake B2 also rises.

Here, in this process, the second motor generator 5 is subjected to theconstant-speed control. Also, a control level of the line pressure isset to a high state H1 in order to raise the line pressure promptly.

When the oil pressure outputted by the electric oil pump 33 risesgradually and the brake B2 is engaged gradually (i.e., when the torquecapacity of the transmission 6 is increased gradually), the speed of thesecond motor generator 5 is lowered gradually and stopped at the endbecause the rotation of the output shaft 2 is halted. The rise in theoil pressure is judged on the basis of the change in the speed of thesecond motor generator 5 (at the time point t2, corresponding to StepS5). Then, the speed of the electric oil pump 33 is lowered and thecranking of the engine 10 is commenced (from the point of time t2 to apoint of time t3, corresponding to Step S6). The speed of the engine 10increases as a commencement of the cranking (from the point of time t2to the point of time t3). When the engine 10 completely starts rotatingautonomously (at the time point t3, corresponding to Step S7), therotation of the electric oil pump 33 is halted (at the time point t3,corresponds to Step S8). Feeding of the oil pressure is carried out bythe mechanical oil pump 32 from then on (after the point of time t3).Additionally, the control level of the line pressure is set to a lowstate Lo when the cranking of the engine 10 is commenced.

Accordingly, in case the oil pressure generated by the electric oil pump33 exceeds the preset value, the output of the electric oil pump 33 islowered and then the cranking of the engine 10 is carried out by thefirst motor generator 11. Therefore, the electric power corresponds to alowered amount of the output of the electric oil pump 33 can be used forthe cranking of the engine 10. As a result, the cranking time can besaved.

Moreover, the oil pressure is judged to excess the preset value at theinstant when the change comes to the speed of the second motor generator5 which is subjected to the constant-speed control. Accordingly, therise in the oil pressure can be detected or judged on the basis of thechange in the speed of existing second motor generator 5.

Furthermore, if the preset time, in which the oil pressure does notexceed the preset value, has elapsed after the electric oil pump 33 isstarted, the output of the electric oil pump 33 is lowered bycontrolling the electric oil pump 33 to the non-operating side, and thecranking of the engine 10 is commenced. Therefore, it is possible toavoid an excessive consumption of the electric power and to start theengine 10, even if there is some malfunction such as a failure in theelectric oil pump 33.

Here will be briefly described the relations between the aforementionedspecific examples and this invention. The functional means of Step S5 orthe electronic control unit for providing the same function correspondsto “the oil pressure judging means” of the invention; the functionalmeans of Step S6 or the electronic control unit for providing the samefunction corresponds to “the electric oil pump output lowering means” ofthe invention; and the functional means of Step S6 or the electroniccontrol unit for providing the same function corresponds to “thecranking means” of the invention. Also, the functional means of Step S3or the electronic control unit for providing the same functioncorresponds to “the second electric motor control means” of theinvention.

On the other hand, the first motor generator 11 corresponds to “thefirst electric motor” of the invention, and the second motor generator 5corresponds to “the second electric motor” of the invention.

1. A control system for a hybrid vehicle, in which an internal combustion engine is connected to a first electric motor and to an output member through a power distributing mechanism, in which a second electric motor is connected to the output member through a transmission wherein a torque capacity is varied in accordance with an oil pressure, and which has an electric oil pump for generating an oil pressure to set the torque capacity of the transmission, comprising: an oil pressure judging means for judging whether or not the oil pressure established by operating the electric oil pump is raised higher than a preset value; an electric oil pump output lowering means for lowering an output of the electric oil pump in case the oil pressure judging means judges that the oil pressure is raised higher than the preset value; and a cranking means for carrying out a cranking of the internal combustion engine by the first electric motor.
 2. The control system for a hybrid vehicle according to claim 1, comprising: a second electric motor control means for carrying out a constant-speed control to keep the speed of the second electric motor at constant speed in the process of raising the oil pressure by operating the electric oil pump; and wherein the oil pressure judging means includes a means for judging the oil pressure on the basis of the change in the speed of the second electric motor which is subjected to the constant-speed control.
 3. The control system for a hybrid vehicle according to claim 1, further comprising: a complete combustion determining means for determining a complete combustion of the internal combustion engine on the basis of a current value or speed of the first electric motor.
 4. The control system for a hybrid vehicle according to claim 1, wherein the oil pressure judging means includes a means for judging whether or not a preset time period, in which the oil pressure resulting from the operation of electric oil pump does not exceed a preset value, has been elapsed; wherein the electric oil pump output lowering means includes a means for lowering the output of the electric oil pump in case a lapse of the preset time period is judged, and wherein the cranking means includes a means for cranking the internal combustion engine by the first electric motor after the lapse of the preset time is judged by the oil pressure judging means.
 5. The control system for a hybrid vehicle according to claim 4, further comprising: a fixing mechanism for halting a rotation of the output member; and wherein the cranking means includes a means for carrying out the cranking of the internal combustion engine when the rotation of the output member is halted by the fixing mechanism.
 6. The control system for a hybrid vehicle according to claim 5, wherein the fixing mechanism includes: a parking gear which is installed on the output member, and a parking lock pawl for halting a rotation of the output member by engaging with the parking gear when it is shifted to a parking range.
 7. The control system for a hybrid vehicle according to claim 1, further comprising: a line pressure control means for switching a line pressure as an initial pressure of an oil pressure to be fed to the transmission, from a predetermined low pressure to a high pressure higher than the low pressure when startup the internal combustion engine while the electric oil pump is generating the oil pressure, and for bringing the line pressure from the high pressure back to the low pressure after a completion of the startup of the internal combustion engine.
 8. The control system for a hybrid vehicle according to claim 1, further comprising: a mechanical oil pump, which is arranged in parallel with the electric oil pump, for generating an oil pressure by being driven by the internal combustion engine.
 9. The control system for a hybrid vehicle according to claim 1, wherein the power distributing mechanism includes a gear mechanism for performing a differential action to distribute an output torque of the internal combustion engine to the first electric motor and the output member.
 10. The control system for a hybrid vehicle according to claim 9, further comprising: a fixing mechanism for halting a rotation of the output member, in case of cranking the internal combustion engine by the first electric motor.
 11. The control system for a hybrid vehicle according to claim 9, wherein the gear mechanism includes a planetary gear mechanism, comprising; an input element to which the torque of the internal combustion engine is inputted, a reaction element to which the first motor generator is connected, and an output element to which the output member is connected.
 12. The control system for a hybrid vehicle according to claim 9, wherein the gear mechanism includes a single pinion type planetary gear mechanism, comprising; a carrier to which the torque of the internal combustion engine is inputted, a sun gear to which the first motor generator is connected, and a ring gear to which the output member is connected.
 13. The control system for a hybrid vehicle according to claim 1, wherein the transmission includes a mechanism capable of switching a gear ratio at least between high and low.
 14. The control system for a hybrid vehicle according to claim 13, wherein the mechanism includes a Ravigneaux type planetary gear mechanism.
 15. The control system for a hybrid vehicle according to claim 1, wherein the transmission comprises: a first sun gear which is fixed selectively, a ring gear which is arranged concentrically with the first sun gear, a first pinion gear which meshes with the first sun gear, a second pinion gear which meshes with the first pinion gear and the ring gear, a carrier which holds those pinion gears, and which is connected to the output member, and a second sun gear which meshes with the second pinion gear, and to which the second electric motor is connected.
 16. A control system for a hybrid vehicle, in which an internal combustion engine is connected to a first electric motor and to an output member through a power distributing mechanism, in which a second electric motor is connected to an output member through a transmission wherein a torque capacity is varied in accordance with an oil pressure, and which has an electric oil pump for generating an oil pressure to set the torque capacity of the transmission, comprising: an oil pressure judging device for judging whether or not the oil pressure established by operating the electric oil pump is raised higher than a preset value; an electric oil pump output lowering device for lowering an output of the electric oil pump in case the oil pressure judging means judges that the oil pressure is raised higher than the preset value; and a cranking device for carrying out a cranking of the internal combustion engine by the first electric motor.
 17. The control system for a hybrid vehicle according to claim 16, further comprising: a second electric motor controlling device for carrying out a constant-speed control to keep the speed of the second electric motor at constant speed in the process of raising the oil pressure by operating the electric oil pump; and wherein the oil pressure judging device includes a device for judging the oil pressure on the basis of the change in the speed of the second electric motor which is subjected to the constant-speed control.
 18. The control system for a hybrid vehicle according to claim 16, wherein the oil pressure judging device includes a device for judging whether or not a preset time period, in which the oil pressure resulting from the operation of electric oil pump does not exceed a preset value, has been elapsed; wherein the electric oil pump output lowering device includes a device for lowering the output of the electric oil pump in case a lapse of the preset time period is judged, and wherein the cranking device includes a device for cranking the internal combustion engine by the first electric motor after the lapse of the preset time is judged by the oil pressure judging means.
 19. A control method for a hybrid vehicle, in which an internal combustion engine is connected to a first electric motor and to an output member through a power distributing mechanism, in which a second electric motor is connected to the output member through a transmission wherein a torque capacity is varied in accordance with an oil pressure, and which has an electric oil pump for generating an oil pressure to set the torque capacity of the transmission, comprising: a step of judging whether or not the oil pressure established by operating the electric oil pump is raised higher than a preset value; a step of lowering an output of the electric oil pump in case the oil pressure is raised higher than the preset value; and a step of carrying out a cranking of the internal combustion engine by the first electric motor. 